I mean at the same time.

What are the restrictions?
Leader positions?
Opposite clans?

It is allowed.

Beyond that, the rules come down to Immortal discretion as to whether you’re taking advantage of outside knowledge to your advantage (jumping between characters, scouting, etc).

Generally the only other issue is that we try not to let one player hold multiple limited positions (leader in different cabals, elite Empire spots, etc), but those situations have typically amounted to requiring one or the other to step down.

Also, for a leader, one of the big criteria is simply being present. If you're splitting time between two (or more) different characters, then it could prove difficult to log enough hours on your leader character to keep the position.

Personally, I recommend playing only one character at a time.

I rolled an Imperial after I had an Outlander. Was going to let Outlander auto, hadn't played it since rolling Imperial, but an immortal sent me a tell saying I had to pick one.

Certain imperial imm likes emperors to be the only characters though.

I wasn't aware of any policy against having characters in two opposing cabals. If one of them were in a leader position that would definitely raise some red flags. Otherwise, I'm a little surprised you were taken aside and told to delete your Outlander.