When you take a picture with a flash, the flash discharges from a bank of capacitors for a given time to regulate the flash power. The higher power the flash, the longer the flash is discharged for, and thus, the more power is used and the longer recharge takes.

The most common solution for your problem is to alter the criteria so that you can take more flashes in a given time. This can be accomplished in one of three ways. The easiest way is to simply reduce flash power. If the flash is active for less time, there is less power that needs to be replenished and the flash will be ready sooner. For example, while the flash I use generally requires a couple seconds between full power flashes, at 1/128 power, it can make 40 flashes in under 1/50th of a second.

If you need a given power level the answer to this problem is either a flash that takes an external charge pack or a flash with better capacitors or more power internally. When I'm using my Canon 600EX-RT, it is limited to about one full power flash per couple seconds, with an external battery pack, that extra power boosts it to less than 1 second per full power recharge (much faster if using lower power). Similarly, since the power of the flash is higher than cheaper flashes, it doesn't have to discharge as much (percentage wise) to get a particular flash, so it recovers faster.

While your idea for alternating flashes is interesting, I'm not aware of any systems that allow for that, at-least not for under the cost of simply buying a flash that would support a relatively moderate speed of 4 flashes per second.