My 10 and 20 gallon tanks have been cycling for a little over a week now and nothing has happened. I put the ammonia up to 4ppm on the 10 gallon and 3ppm in the 20 gallon. I then added some tetra safestart to get the cycle going.
A week later I tested both tanks and there were no nitrites. I have read the cycling stickies and there should be some nitrites by now. I tested the ammonia in my 20 gallon and now it's gone down to 0.5 but since there's no nitrites or nitrates, it must have just evaporated or something. I will add more ammonia, but what am I doing wrong in my cycle?
My 10 gallon heater is currently in use, so both of my tanks are in the low 70s to high 60s. I can turn on my 20 gallon heater if that will help, but I'm using the heater for my 10 gallon for my betta in his temporary 5 gallon. Is there anything I can do to help my cycle?

Usually cycles can take up to 6-8 weeks to fully stabilize and mature, your phase right here is completely normal. Every cycling experience is different, so you can only base it off your experience.
Just keep on adding ammonia to 4ppm once it's near the 1ppm mark. Just wait a while, and then your nitrites will show up sooner or later. I don't think evaporation can cause a huge nitrite downfall, either. Bacteria LOVE warm places, they also like a lot of O2, (oxygen) a filter on high, and dark places.

Good luck, a very experienced cyclist called Hallyx is here, that member can really solve your problem. (Is cyclist even a word in terms of tank cycling? Lol)

Hm... My 20 is currently cycling but I'm in the nitrates phase..
I think after the 2 week mark you should start worrying. My nitrites appeared around 1 1/2 to 2 weeks after a few doses. Remember to not let the ammonia go over 6ppm. May I ask, what's your pH?

You need to keep feeding the bacteria. You want to test daily and keep adding drops. You can't just add ammonia once and let it go. It's also too cold. You need to add a heater and bump it up to 80. Adding an airstone will also help. Here is a good guide on it. http://www.bettafish.com/showthread.php?t=47838 Even with the SafeStart it will probably take at least 3 weeks to fully establish. It could take as long as 6.

I added more ammonia and put a heater on the 20 gallon so that should help. There's nothing I can do to heat the 10 gallon though since my heater is in my betta's 5 gallon. I'll wait another week and see if there's any nitrites.

Good advice from the others. I will add that if you use a bacteria booster the nitrites may not show. I used stability in my 46g & I showed nitrites once & only once. I have read others say they never saw nitrites. Only test your water every 24 hrs & only dose ammonia every 24 hrs. It's only been a week so don't stress over it & IMO your Ph is perfect at 7.6 but keep checking it because if it drops too low, low-mid 6's, it can stall your cycle.