yes you can use any security checkpoint for any flight, there are not really two "terminals" but two departure halls and three luggage claim areas and after security it's just one area - perhaps not even signs saying T1/T2 (because T1/T2 are really departure halls and not terminals).
There are some new gates at the end of the T1 area which are now used by finnair domestic flights, so if you stick to airport authority terminology you would check in at T2 and depart from T1!

sometimes the upstairs priority security area in T2 is also open for non-priority customers although i'm not sure there is any way to find out when they are doing this apart from go up the escalator and see for yourself.