Making a support marksman would be tough, since traditionally, marksman are all about sustained damage, scaling into the lategame with lots of gold, and needing peel to survive, while supports are supposed to be strong early game, function on a low income, and provide the peel that marksman need.
A marksman is focused on taking down big, tanky targets like tanks, buildings, and objectives, which means that they need to scale into the lategame when taking those things is most important. Supports with carry potential, like Brand and Lux, also scale into the lategame, but are balanced by high mana costs early on, which doesn't limit autoattacks.
In order for it to work, they'd likely need abilities in their kit that empower their autoattacks so that A) Their ability to function like a marksman is gated behind cooldowns/mana and B) they don't need as much gold to get stats.
Honestly, they would probably need to be an AP marksman, like Azir, so that they can't just build AD and hit stuff, they need to use abilities to get access to the damage they've built. That way, enemies are able to play around their damage and they can't just be the marksman. A passive similar to Pyke's, that converts AD/AS into AP, which their abilities then turn into on-hit damage/attack speed stims, would be one way to do it.
The main problem I see is that a team usually doesn't need two marksman. Unless the intention is for them to run as a non-traditional botlane (e.g., the marksman-support is supporting an assassin, mage, or fighter, rather than another marksman), most teams really wouldn't want two squishy teammates that both need time to scale.
