if you can make some kind of obstacle to go on the track that 1/8th buggies can't get past, but 1/10th will have no trouble with, that would be a perfect solution

i don't know how much the height difference between 1/10th and 1/8th buggies is, but if you can put a bar across the track at such a height that 1/8th will hit it and 1/10th will miss it, then can take it off for race days, that might work? That way 1/8th couldn't use the track at all, and 1/10th could practice whenever they wanted (albeit with a slight inconvenience i guess)