I have a mesa dual rectifier head and an old mesa dual caliber dc-5 head, I want to play out of both of these heads at the same time for different tone ranges and such. The old dual caliber has a slave output (or input I don't really know, just says slave and has a knob next to it) on the back. I don't know much about slaving and I have tried a A/B/Y switch but there's too much hum from ground issues and I'm not really interested in that method. Also, do I need two cabs? Sorry if these are dumb questions I have looked around the forum and I can't really seem to find the explanation I'm looking for. Thanks guys!
Two cabs or each side of a stereo cab.
Gilchrist custom
Yamaha SBG500
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100

Cathbard Amplification
My band
Okay, so how would I hook up the two heads in order to play out of both of them simultaneously? Would I run out of the effects loop or something on my dual rec into the slave on the back of the dual calibur??
Just listen to Cathbard.

You can't do what you want with what you have. It sounds like you've already tried because you refer to an A/B/Y switch.

The Slave Out on the Dual Cal is for, I believe, taking the preamp signal and slaving it out to a power amp. That is not going to give you the two amp awesomeness you are looking for.

I am assuming you only have 1 speaker cabinet is that correct? Can I also assume that is not a stereo cabinet?

Generally speaking amp heads need to match up with the speaker cabinet pretty closely. There are exceptions to this rule but just to keep it simple:

Let's say you have a 16 ohm 100 watt cab. Then you want a 100 watt amp set to 16 ohms.

(again, there are exceptions to this so all you amp nerds can back off)

Now, if you try to run both amps into that 16 ohm cab then you create several problems. There are products out there that can help in your situation, like a Tonebone Headbone, but what you really need is a secondary cab for now and keep it simple. Then your A/B/Y pedal will work. You should be able to find a decent 112 cab starting at $100.

This is a good reference point with little diagrams.