Yes you can Kenkay i look at the visa as just making me legal during my stay in Turkey. You can have visas overlapping if need be, if one is going to expire whilst in Turkey you can purchase the second one to exit on but are meant to get the new validated on entry to Turkey, it is defienetly all about not exceeding the 90 days out of 180 days...
Sleuth that is a great idea I had not thought of doing that rather then go down the residency route, I thought in the beginning of the new visa rules that once you had taken 90 days you had to vacate Turkey for 180 days but it is only 90.... Bit late for us this year but I shall bear this in mind for next year...