If both amps are identical, then vertical biamping is usually the best way to go, especially if the amps have a single power supply feeding both channels.
In this case, one channel feeds the lows and the other feeds the mids/highs and all the muscle of the power supply is available for the channel feeding the lows, which can be quite a big advantage in terms of effective power.
Another advantage of vertical is that the speaker wires can be very short, if you wish.
With identical amps used vertically, the effective power available is slightly more than doubled unless the amps have dual mono power supplies, in which case much of the extra power available from the channels driving the high frequency section is wasted and the apparent power available is similar.
I think this is a waste of time and money - you would be better off buying a better amplifier, or an amplifier dedicated to HF duty and more suitable to it like, maybe, tubes or low power "class T".
The key point is that it takes vastly less energy to power the HF section of a speaker.
In any case, you will need 2 Y-splitters of some type for the experiment - it could be a short female RCA to 2 male RCAs if you wanted to use a single interconnect to two amps used vertically, with the split happening at the amp. They could also work to feed two amps in a horizontal set up, if the amps were stacked.
For experimental purposes any old cheap Y-cord will do the job, to be replaced by something decent once you determine whether you want to commit to a configuration.
When I was bi-amping my studio monitors a few years ago, I took the top off my amps (Sugden Au 31s) and soldered an internal jumper so that the left input was sent to both channels. This simplified wiring - I could just use a single interconnect to the amp.
The improvement was massive, by the way, and I then found out exactly how much it means to have a surplus of power, since, theoretically I already had more than enough for the levels I was listening at and the sensitivity of the speakers.
I was using Tannoy Arden speakers which are 15" dual concentrics and run around 94dB/W/m. The amps were rated at about 120 watts per channel, I think, and I listened at 6 a foot distance, so the headroom should have been plenty and yet when I vertically bi-amped, the extra ease was very obvious at any level above about 80 - 85 dB. I sensed that I could gain further advantage by doubling the power still further.
