Is it possible to use two battle systems at once in a game? Say for example, if I wanted boss fights or storyline based fights to be in the original rtp battling system, could I event that whilst still having maps with an action battle script on them, so that enemies can be fought in real time?
Please help me with this!

In further detail, Id like to have an ABS that uses the keys, spacebar, numpad (skills) etc. and then have a boss fight occur in the next map in the standard forward facing rtp style. Is this possible?

GubID tactical battle system can be used alongside another battle system. I think it is the only one that doesn't overwrite the Default Battle System.

This is an interesting idea, however be careful, it could get rather complex and difficult.

You can have as many battle systems as you want. I programmed a few screens of crude ABS using touch system targetting, had a very overlapped side-scroller, and at one point had something attack the screen Earthbound style (very strange code, since the gameplay was still technically side-scroller, and I was effectively "attacking the Player").

The key is making non-intrusive code. If I were to truly switch over to frontal (Eartbound) system, all I'd need to do is create extra blank characters and position them at the bottom or center of the screen. Voila, swap out the old characters and insert the new, and you have a new system.

Sorry, what exactly do you mean by swapping characters?

From a theory and ideas perspective, and ignoring the technical scripting, yeah, it's extremely common to have multiple battle systems in a game. Suikoden games, for example, generally have three battle systems: normal battles, one-on-one duels, and massive tactical battles. Other games might have secondary battle systems used for sports, minigames, pet battles, arena matches, or something else.

The general method is to have one "main" battle system, and other less detailed, generally less enjoyable battle systems that are only used in a few specific situations. They don't have to literally be minigames - they could be used in major plot events. But typically they're designed with the assumption that the player is only going to use them a limited number of times, so there's just not a lot of room for growing and strategizing. The challenge in these kinds of secondary systems is usually for the player to simply learn how they work, rather than to overcome specific obstacles with strategy and skill.

They add some variety to the game and help change things up. However, some players dislike these kinds of totally disconnected secondary gameplay systems, because it's not the game they signed up to play. This is especially true if it's done poorly, but can even be the case if it's done well if your main battle system is fun enough. The system being mandatory also increases the likelihood of it being a huge pain in the ass, but even just offering rewards that make you stronger in normal battles is enough to make a lot of players feel compelled to do it no matter how miserable it is. In Kingdom Hearts games, for example, I pretty universally hate all the minigames and alternate gameplay systems, and the fact that the first five hours of Kingdom Hearts 2 are nothing but mandatory minigames doesn't help at all, while Kingdom Hearts 1 forces you to do that stupid gummi ship garbage before every stage and to get five stars on every gummi ship stage for the best ending.

There are exceptions, of course. I know some people spent more time playing Blitzball in FF10 than playing the real game in FF10, making it an arguable exception to both of my above observations. But if your second battle system is really that good, I personally wonder why you wouldn't use it as the core battle system for a whole game.

It all comes down to how much attention you'll give to each battle system. To keep things interesting, give equal attention to each battle system, like every boss battle or so change the battle system used. Make specific items and concepts for all of them so they all have depth.

I was originally planning on making two battle systems for my newest game, but I'm just fine with one myself. But I digress, like others said, have as many battle systems as you please, but give them all substantial attention.

In rpg2k3 (which is what I use, but you can adapt this, given sufficient knowledge), they have Heroes (let's call them David, Joey, and Elena). These guys have battle sprites, like a side-screen graphic or whatever. Before the battle, you switch David, Joey, and Elena for heroes with identical names and normal sprites, but different Battle Sprites you've edited (basically, just blank sprites, using an actual blank picture file, as no file produces an error). Instead of Hero 1,2,3 for instance these might be Hero 17, 18, 19. On the Battle Layout menu, where you can place all "Character" events on the screen (that's the term it uses), the new heroes' Battle Sprites are blank for all animations, and you simply move them to the center of the screen.

You position the enemies for this battle at the top of the screen, and have them be facing the screen. Any attack then looks like it targets the screen.

You can switch this back after the battle, simply by changing the heroes back to the ones with the original Battle Sprites.