Avoid taking large doses of both, as severe confusion is likely to set in along with a greatly increased propensity for delusional thinking (especially delusions of grandeur and persecution, as well as ideas of reference). Those are two places you don't want to be visiting at the same time -- the risk of psychosis may be high in the inexperienced. Also be aware that cannabis strongly potentiates both DXM and LSD, so if you do intend to smoke, it's recommended to wait until the peaks of both drugs have passed and to carefully moderate your intake. Other than that, I have survived large-ish combinations of the two (say 800mg DXM + four strong blotter of LSD, with some tolerance to both). These experiences are not for the faint of heart, and a trip sitter is strongly recommended; higher doses in combination may present acute undesirable side effects.
I always preferred to dose the DXM after the LSD. That way, you get to experience the peak of the LSD and later supplement it with the DXM. Taking a recreational dose of DXM beforehand is likely to "taint" some of the core psychedelic experience of LSD with the stoning dissociation of DXM. YMMV; ask yourself if you want a wonky LSD trip (unpleasant, in my experience) or an LSD trip that leads up to a psychedelic plateau of DXM dissociation (bizarre, but very rewarding in my experience).
