If i index two columns (indepentently) do i get the same performance as putting and index on the two
Your question is a bit jumbled.  No, you can't get theoretically the same performance, unless one of the two columns is unique or nearly unique.  But yes, you can get performance that while slower than the composite is still much faster than no index at all.  Will it be fast enough that you don't need the composite index?  No one can answer that but you.  We don't know either how fast you need it to be, nor how selective the condition against each column will be (both independently and jointly).
But if you are going to have indexes on (a) and on (b), you might as well just change the first one to be on (a,b) or change the second to be on (b,a).  The extra cost will be nearly negligible (unless the added column contains wide data values) while the extra value at least has the potential to be large.  The cost might be slightly higher in v13, because having the composite index might defeat the new deduplication logic compared to the one with just the prefix column.
Doing a detailed analysis of whether it is worthwhile is probably not worthwhile.