The army you have chosen for now is specifically Chaos Demons. Chaos is a bigger faction, which the demons are part of.
You can ally yourself with other chaos factions to build a larger army, but these other chaos factions are different in 40k and AoS. Only the demons are the same in both systems. So if you truly want to play both systems, this will put a limit on your choices.
As for being able to play both 40k and AoS: How useful is this to you? Are you really planning to play both systems?
If you are going to play with the same group of people regulary, chances are they will prefer one system over the other, because their chosen armies are not able to play both.
Even if you do plan to build an army for both systems, a Demon army that is good in AoS, is probably not going to work quite the same in 40k. You may want to take the build of your army in different directions, depending on what system you play in.
I think you should first decide if you really want to commit to a dual-system demon-army and then go from there, because this has quite a few implications.