It should be fine. Strictly speaking, syrian rue isnt toxic. Those other alkaloids get a bad reputation just because they cause things like ataxia and abortion. In quantities near to normal doses they are not toxic and 5 grams of rue taken at once is commonly regarded as the top end of normal doses. Iranian women take 9 grams in one drink to induce abortion.
I've never taken a full dose of syrian rue 3 hours after another full dose but looking over my tripping journal, when I was experimenting with rue tea timing, on multiple occasions I dosed a total of 4.5 grams within 30 minutes and my body weight is just 54 kilos. I got no toxic effects, although that dosage did leave me walking like a drunken sailor from rues ataxia and my thoughts werent quite as clear because of the heavy harmaline dose [I know that part was harmaline from later experiments].
If in doubt you can do a test run, using only rue and no DMT, to prove to yourself its safe to do with DMT. It'll probably just make you feel a little drunk for 8 hours or so.
If you want to cut the dose down to minimum another experiment could be to take 3 grams rue, do nothing for 3 hours, take an additional 2 grams, and follow that 5 minutes later with a very low- but known active- dose of DMT to establish if a T+3 hour 2 gram redose is enough to activate a DMT dose. This is what I would do if trying to minimize rue dosage while avoiding potentially wasting a full DMT dose in the middle of a trip.