Would spinning the coin get rid of that bias?

D8? D20/5 x d20/10

Am I missing something here? Can this even generate 5 or 7?

D20/5 gives [1...4] and D20/10 [1...2], of course assuming whole numbers. Where to get the factors for 5? 5 can be factored only as 5x1 or 1x5 and the 5 cannot be found either in d20/5 or d20/10. Same is true for 7.

And I don't see it happening either if we allow rational numbers. To get 5 we would get the following expressions
5= d120/5 x d220/10 = d120 x d220/50
or 250= d120 x d220
And two d20 multiplied together cannot give us 250.

Math baby?

To keep the same probabilities, you can only reduce and only to one that is a factor. E.g. d20 can be equivalent to d10, d5, d4 and d2.

Multiplying the rolls messes things up. As an example, for d12 as a d6xd2 you have double the chance to roll 2, 4, and 6 and no chance to roll 7, 9, and 11.

You could make the equation a little more complicated (6×(d2-1))+d6 to make it work.

I always divide by two and round up for d3

But if you roll the d00 on accident, you can easily still treat it as a d10. If you roll the d8/4, you can't.

what do we call the other one?

A golf ball?

d% is what I usually see

I've heard it called a tens die or a percentile die. D100 is usually saved for the actual 100-sided die in my experience.