Tips on helping 3.9 y.o. and 12 m.o. to play without fighting?

[AngelDog]

Thankfully my boys get on really well together most of the time. But DS1 wants to spend his time assembling wooden track and driving trains on it, and all DS2 wants to do is take the track apart. Result: pushing, biting, general strops from both of them.

Unfortunately the room where they mainly play is not where I spend most of my time (the kitchen). We're doing an extension to fix that problem, but it won't be done for a few months. I simply can't manage to find any more time to supervise or take them out due to various factors. There isn't room for them to both play in the kitchen, and the trainset would be under my feet anyway.

We have a stairgate on the living room door and give DS1 plenty of time to be in there without destruction from DS2 but the poor baby wants to go in and play there sometimes.

I've tried a HTT approach of problem-solving with DS1, and his ideas are limited to killing DS2 and permanently banning him from the front room.

Tips?

Watching with anticipation as I have exactly the same problem with my 4.5 and 19 mth old. Keeping them apart is tricky as the younger one always wants to get involved with whatever the older one is doing. Sweet, but inevitably ends with pushing/hitting/screaming. No doubt this is perfectly normal, but a bit draining if nothing else.......

Having exactly the same problem this morning.
Set up a big aquadoodle,etch-a-sketch and a colour wonders set for non-messy creative time. Eldest 3.6 wants to draw around objects whilst youngest 19m keeps stealing the pen and crying. Making Eldest cry with frustration.
Driving me mad