One turntable with two arms, or two turntables with one each - which would you prefer?

How about one TT, one tonearm, one cartridge but with two styluses, huh? 
bet you never heard of one cartridge with two stylus, one MM and one MC

LOL!

For a question like this I must defer to Harry S. Truman.

He always said he preferred the one arm approach.

Particularly when speaking with economists.

It avoided the dreaded words "but on the other hand".

I use a VPI with a uni-pivot and two arm wands. Once set up I switch the wand and oly have to adjust the vta.

IMHO a silly question, unless you are rolling in money.

The best way to go is, one of the best turntables you can afford and one of the best arms you can afford with one of the best cartridges you can afford.

Do not buy two fair set ups at the expense of one excellent one. If you have to spend some time setting up cartridges, so be it. You are cheap labor. 

There are usually limits in terms of space so, two arms on one turntable is the most space effective and also the above argument still holds, one great table vs two not so great tables. 

He claims that just mounting a second arm would degrade the sound quality of the first arm.

@thekong I don't have a multi-armed TT, but it sure seems like the unused arms could pick up airborne vibrations and transmit them to the plinth.  I always wondered why this is rarely, if ever, discussed.