How can I create a dynamic multiple choice that uses selections from another dynamic multiple choice.

That is an interesting question. I'm not sure why you need another dMC showing the selection of a previous dMC, but what the heck why not do it. Try:

let selected := numbers(MC1);
select Table1
    where (var v := this;
    count(selected[= v]) > 0)

or

let selected := numbers(MC1);
select Table1 where contains(selected, number(Id))

replace:

MC1 = with dMC A

Table1 with the table name that dMC A is based on.

Here is a third option that does not use the dreaded select statement.

let selected := numbers(MC1);
for loop1 in selected do
    record(Table1,loop1)
end