If any thing wait longer for the dxm, you just put your heart in some strain with the cocaine now just kick it a little. I am not really sure of the dangers but i would say it is wise to wait for next day to dxm.
You don't want to go into a DXM trip while coming down from Cocaine. I do distinctly remember that Amphetamines + DXM can cause serotonin syndrome but I'm not sure if Cocaine can. It can put quite a strain on your heart to mix the two though.
As far as I'm aware Cocaine is a triple reuptake inhibitor so I'd assume it has at least some action on Serotonin but it's not significant. If I was you I'd wait but I can't imagine you'll have too much trouble if you insist on doing it.
I once prepared myself for a hideous cocaine comedown by purchasing enough DXM for a low third-plateau experience, but stupidly could not resist the urge to take it before even my first line.
It took approximately an hour to kick in really noticeably, an hour during which I had done 3-4 good lines. I first noticed the effects of the DXM when the person across from my suddenly looked like a giant sitting a mile away, someone large yet distant but appearing to be close and small, off by just enough for me to detect the difference.
The night progressed as I downed another gram over 3 hours. When the DXM peaked I experienced a strange "teleportation" by which I would black out only to become lucid upon doing a line. Basically I would suddenly appear in a different part of the room having done another line which brought me back to reality.
Overall the experience was terrifying, though I was told I acted completely normal during the blackouts, or rather normal under the circumstances.
I experienced no issues with my heart rate and the next day had a regular DXM hangover.
Cocaine's major MOA is its function as a dopamine reuptake inhibitor, though it also blocks the reuptake of norepinephrine and serotonin. DXM blocks the reuptake of norepinephrine and serotonin alone, hitting sigma and opioid receptors too (and of course NMDA). I experienced a high dose combo, with large doses of both drugs, with few ill effects. This is a meaningless anecdotal report but I figured I'd share. High doses of DXM which may have you tossing in your bed in and out of lucidity can be a recipe for blackouts with a get-up-and-go stimulant. Overall I feel lucky that I behaved during my blackouts, and feel that the combination of such a relaxing, removing drug and such a powerful stimulant is not smart, though it may be safe. I would call it unsafe though for anyone prone to panic attacks or with heart issues. I'd also recommend never using a powerful dose of DXM. YMMV.
Both drugs have complex effects on the body, with DXM being less understood. A healthy person would probably have no ill effects befall them, but with the risk of blackouts it's advisable to avoid this combo. Ketamine seems to have a larger history of safe use with cocaine.