" If I was to power two long bows at once, would you recommend a dual output power supply or just one larger power supply? "
The only benefit of a dual output power supply is that you would be able to control the temperature of each wire individually, but really foam cutting rarely requires that level of precision. The other problem is that many dual output cutters are a lower overall amperage supplied to each heating unit.
Meaning.. For the money, you could get a single output 10 amp power supply, or a dual output that supplies 5 amps to each output. Granted, many dual outputs allow you to power a single output at full capacity, but what's the real added benefit of dual output? One may argue not having to change cables between cutters, which I suppose is personal preference.
If you simply place your cutting devices in series, the power supply just sees it as being one long cutter. The amperage requirement does not change for heating the wire, but because it is a bit higher resistance you just have to increase the supplied voltage. So ensuring that your cutter has enough voltage to power both cutters is also important. But again, most cutters only require about 10v at the most.
if you place your cutters in parallel, it will throw off the dynamic of the resistance circuit and alter your power supply requirements.
Additional information regarding multi-cutters can be found on the website ::
( http://hotwirefoamcutterinfo.com/Hot_Wire_Foam_Cutter_Info/_MultiWireCalc.html )