Do we use plural of percent when having two percentages? [closed]
No. Seven and eight percent respectively is a perfectly normal way to compare percentages. Here's another example (that I've just made up):
The quantity of lead in municipal drinking water was measured in Paris and Rome. The concentrations were found to be 3 and 5 parts per billion, respectively.
Percent can be alternately written as [parts] per hundred. One would not write parts per billions as it does not make sense (how many billions?), nor would one write parts per hundreds.