Actually, I had that idea also - what you could do is have a whole lot of solar powered mass drivers that freeze the CO2 and nitrogen to dry ice, cover it with a reflective cover - and then fire those pellets in a stream towards Mars.
You'd have far too much for Mars - so - you could also use it to supply the asteroid belt also.
Don't see this happening any time soon. You'd also have the problem of spinning up Venus, and of supplying water to it.
And as for Mars - is a huge amount more needed for Mars than just getting enough atmosphere.
Also another idea - is to supply huge amounts of water to Venus - enough to convert all the CO2 except an Earth amount - to organics. Also help lubricate the crust and maybe get continental drift working more like Earth instead of the global resurfacing every few hundred million yeas you get on Venus at present.
Maybe a few centuries from now though our descendants might have the knowledge needed to do things like this? And the power - through fusion power or abundant solar power or both? Or other power sources our physics simply can't describe yet....