Do I need to apply for a Thai visa before my trip if I will be entering Thailand twice for a total of 5 weeks?

PLEASE HELP?! :(

Hi Guys, sorry to bother you but I'm in need of some urgent advice. My fiancé and I are travelling to South East Asia in just over 2 weeks- we are staying in Vietnam and Cambodia for 18 days and then in Thailand for 2.5 weeks before heading to Laos for 2 weeks then back to Thailand for 2.5 weeks before heading home. I'm so confused on whether or not I need to apply for a Thai visa before I go (which doesn't give me a long time) or whether we get our visa on arrival as we're entering twice and won't be there for more than 21 days at a time...

What do I do?!

Thanks in advance!!

Kia x