Strapping the amps to make them (one amp) would let you get back down to the 1 ohm load that you wanna be at with your particular amp. When people ask for GOOD CHEAP things, let's just get easy money.. And burn fat while were sleeping. Sorry bro but just take a deep breath and buy a DC power, mechman or something as your alt. do your electrical, I'd go another battery or 2 (total of 3-4) and a upgraded alternator, 240+ amps. 5k watts is one of those numbers where it takes A LOT more to support it.
Kinda like a 600 watt system vs a 1200 watt system, there not too far apart as far as power goes but one you can run almost off ALL stock stuff, and the other you need to do significant electrical upgrades. Your at that level, anything over 2500 and it's a big step.
Make sure your electrical is up to snuff and then buy the 2nd amp and enjoy.... OR buy the second amp and just run it at a rediculously high ohm load (to not hurt anything, maybe 4 ohms each amp) until you get your electrical anyways.
The moral of the story is, it takes power to make power bro.