You don't necessarily need to use the same amp, but it should have the same gain as the other. If the amps have different gain, the output from the biamped speaker sections will be out of balance. If the gain is not the same, it would be advantageous to have some way to adjust the levels between the two different amps.
The biggest thing to consider is that if you want more power, you need to buy a more powerful amp. When you biamp in this manner, the watts do NOT add together - this is a very common misconception.
In a passive biamp setup, your maximum output/clipping level is essentially no more than that of the lowest-powered amp. This is because both amps must reproduce the entire voltage swing of the full range signal.
Using 8 ohms speakers and 100wpc amps as an example, I think you'll see what I mean.
To produce 100 watts with 8 ohm speakers requires about 28 volts. So, that is about the limit of what you can get out of the amp, 28 volts. If you biamp with another 100 watt amp, that second amp still has the same 28 volt output so there isn't really any increase in useable output.
However, you were to get a single 200 watt amp, then your output could go to around 40 volts. So, as you can see, a singe 200 watt amp provides a meaningful increase in output vs. two 100 watt amps in passive biamp config.
My advice? If you want more power get a more powerful amp - at least twice as many watts as you have now. Don't waste your time and $$ with a passive biamp setup, spend both on a more powerful amp.
