No, you can only import once. Importing a second set of donations from a second account will either fail, or erase the first set. You can either combine them manually now in one account, or import from one account and then add the others manually into turbotax.
Some clarification here:
1) Last year's "2015 Online" software had a major problem, and wouldn't download any ItsDeductible data from an ItsDed account that had a UserID different from the person's Online tax account UserID login. Whether the developers have fixed this for 2016 won't be known until it actually starts up for 2016 and some of us can test it.....and if not working, "maybe" we can get it fixed by January or February
2) I tried downloading two separate ItsDeductible accounts into the just released 2016 DESKTOP Premier software. and the two different account downloads worked OK and didn't overwrite each other. Not sure what happens if both accounts donated to the same-named charity....but my two accounts had one with "The Salvation Army", and the other slightly different with just "Salvation Army" and they came in as separate entries. It might be a good idea to make sure the Charity Names are just slightly different to make sure there are not any overwrite issues. Like the above, or "American Red Cross" in one ItsDed account, and Am. Red Cross in the other.