If you have a paying chess.com membership, then you can go to your settings and allow multiple games to be played at once. Then I'd imagine, just set up several computer games as you normally would. Now you have a simul going 
Also, make sure to give yourself A LOT of time because computers (even weak ones) move as if premoved (so they can't lose on time). Starting a simul with short time control would then be suicide for you.
Also, I think that could be fun practice for a real simul. Most games I ever had simultaneously (in real life) was 3 games against chess friends (I won all 3 
).
p.s. You may be surprised how many real people might be interested in a simul - even on chess.com. Why not message some chess.com friends and see if any of them would be motivated? Even I'd be interested in playing 
Is there any software which I could use to play a simul against weak computer? I know there is no way to get many real people to actually agree for a simul, so a weak computer would be easier. Maybe not on this website but on a different one? thanks