I take 2 lo dose aspirin daily. at times i take 800 mg motrin 3 x day for knee or back problems for 2 to 3 weeks at a time. is it safe to take both?

Aspirin vs Ibuprofen: Check out http://www.Drugs.Com/interactions-check.Php?Drug_list=1310-0, 243-3197. There are concerns that Ibuprofen and other nsaids can mitigate the benefit of aspirin. Talk to your doc about your Aspirin dose as we typically only recommend 75-81mg daily to minimize bleeding side effects. This can be magnified by regular use of Ibuprofen so ask your doc for other options. Consider ppi + nsaid.

Yes: As long as it is safe for one, the other is fine. Especially short-term.