Is a laptop considered a carry on on an airplane when you already have a carry on?

Depends on the carrier.
Some do some don't.

Most airlines allow you to carry two separate items on board - which typically means a small suitcase plus a briefcase or carry bag of some sort, and your lap top would be in that briefcase. But if you mean that you want to carry 3 items on board, then you may well get stopped by the agent at the boarding gate. Best to figure out how to cram the lap top into your 2nd carry-on to get past the boarding gate agent.