Anyone know how to do this please?

I WAS going to buy an Express extension cab for the Express but now I have decided to buy my old F50 back and use the Express for clean, and the F50 with my 6 overdrive pedals.

Using either "amp" is easy because its just an A/B switch BUT...

I want to choose to use either amp (Express 5.50 or F50) with BOTH speakers all the time, so its "like" running a 2 amp head A/B system with one 2x12 cab (actually the 2 combos of course).

Of course to use one amp into both speakers you use the 2 4Ohm outputs in the amp just like using an extension cab, BUT how can I switch to either amp outputs into both speakers.

So I can choose to either use the 5.50 "amp section" into both 5.50 and F50 speaker combo speakers, OR the F50 "amp section" through the 5.50 and F50 combo's speaker.

driving me mad, lol. :?

Seriously? With the quality of the 3&4 channel amps out there today why would you even bother with a two amp setup?

Get a Mark/Recto/Roadster/RKII and forget about the F-50.

My opinions of amp sounds here are just to do with getting the sound I want, when I say the sound is useless, or crap with pedals etc, I mean its no good for me, not "no good full stop".

I can't lift anything heavier than a 1x12 F50 or Express but I would like a 2x12 set up.
No other amp that I have tried can give me the sound I need, I have tried nearly all of Fender's line of valve amps, and other makes, the express that I bought sounds fine clean but really crap with pedals, the F50 is the only amp I have tried to give me the pedal sound I like. I tried the Mesa Rectoverb and tremoverb = both completely wrong for me as the sound was not at all suitable for either "my" clean or overdrive sounds, plus the tones constantly needed readjusting with even the slightest volume change, the tones didn't work for me would need to completely E.Q. it with a graphic, I owed a Lonestar classic 2x12 for a day which also was a big tone disappointment and went back the next day, (one of 4 Mesa's faulty out of the box straight from Mesa UK)
It is common to use 2 amps anyway, some pro's sometimes use 2 identical heads through a 4x12 EV loaded cab, while one song is being played the tech is adjusting the other head ready for the next song, Mark Knopfler did this in the 80's with 2 Mesa Boogie mark 11B heads, and in the 90's with 2 Soldano SLO 100 heads, these days he doesn't use 2 amps the same he uses one for a Marshall sound "Komet" head, or "Reinhardt" head, AND a Tone King Imperial combo for the clean Blackface Fender sound, so again 2 amp set ups.
I was a user of line 6 products until recently because I like to use completely different sounds.
I was hoping the Express would give me both a good clean AND the sound I had with pedals in the F50 but it sounds crap with pedals, if the express sounded like the F50 which I hoped it would I could have used the Express for both clean and overdrive pedals, and bought a Marshall Haze for vintage Marshall sounds. But I have had to buy back my old F50 to get my favourite overdrive sound back, this is a totally different type of sound to a Marshall sound, I use lots of different types of sounds so it can a problem.
I find the Overdrive on every amp I have tried almost unusable for me apart for a few amps like an old Marshall JCM800 combo which when used on a mild crunch sounded nice with a Les Paul. But again although this Marshall sounds great for a very low gain edge of break up clean used with a Les Paul, and probably great with a Overdrive pedal, Dejavibe pedal and Wah for a Trower sound it would still be useless for my main overdrive sound, which I can only get with the F50 and overdrive pedal, I also use 10 different guitars.

If you insist. 1x12 cabs are far cheaper than switching systems. Why not just add a cab under each amp and pick up an A-B switch? Light, cheap and easy to carry.

If you haven't already solved this or otherwise moved on:

Leave the speakers hooked up to one amp's output, and switch preamps. Run FX send from each amp to an A/B and the output to the F50 (or 5:50 if you like) FX return. Then you have a spare power amp handy in case the unthinkable happens.

Radial Cab-bone.