The myth that mixing stimulants and depressants is dangerous to your cardiovascular system is, well, a myth. The heart isn't a piece of string which snaps if you try to pull it in opposite directions, instead it just adjusts it's rate depending on the messages it receives from your brain, which depends on the quantity of stimulants in your system v. the quantity of depressants in your system. More uppers, HR goes up, more downers, HR goes down. Generally speaking anyway, there are exceptions, namely alcohol, which is cardiotoxic (especially in combination with cocaine, as it causes the cocaine to metabolize into cocaethylene, which is more cardiotoxic than cocaine by itself) and increases your heart rate, instead of decreasing it like most GABAergic drugs. But generally speaking, the heart is not a tug-o-war rope, and if anything, taking downers would lower the cardiotoxicity by lowering the heart rate.
That said, it's not a great habit to get into, generally speaking, but that's because it can lead to becoming dependent on the downers, or mixing uppers & downers which can lead to ODing on one or the other (with benzos anyway, it's hard to imagine either being an issue with seroquel, but I've always found seroquel to be overrated as a sleep aid and generally nasty).