I have a Aqua Marina Atlas 12'x33" which is very similar to the Monster and IMHO it would not perform to well with two full size adults on board. By the specs it will handle the weight but I think it will ploughing it's way through the water. I paddle mine with my 8yo old son on board with a combined weight of about 90kg and I wouldn't want to go any more, not if you want to retain any performance at all. The Aqua Marina boards are good quality for the money and I've heard that Aztron are a slight step up in quality but have never owned one personally.
rivertooceanadventures.com.au/products/new-2019-aqua-marina-atlas-sup-paddle-boardrivertooceanadventures.com.au/products/aqua-marina-monster-inflatable-paddleboard-sup?_pos=1&_sid=5e38b7a8e&_ss=rI think a dedicated tandem board is really the way to go if that's what you want to do. The Aztron Galaxie is probably the cheapest way into that but still decent quality. I think the 34" width is fine, it's just the extra length you need both for more volume to support the weight and for two adults to actually have the room to swing their paddles for the stroke and swapping sides.
rivertooceanadventures.com.au/products/aztron-galaxie-16ft-inflatable-sup-paddle-board?_pos=1&_sid=35d2a3a60&_ss=rWhen you say 'moving around the board when underway' I'm not sure what you are planning on doing but it's not an aircraft carrier. Having said that, 34" is very wide in general and they don't get much wider than that. Depends on your balance but any movement from the other rider will upset the applecart especially if the other person is not ready for it. If you've ever paddled with a child or large dog on board you will know what I mean. But it's something that you will soon work out once you start paddling together.
Cheers, Damo