The word subtract comes from the Latin verb subtrahō, whose conjugation gives subtrahendus as the future passive participle ('which is to be removed'), and thereby subtrahend in English. The English word minuend is likewise derived from the future passive participle (minuendus, 'which is to be made smaller') of minuō. Similar derivations include:
- integrō > integrandus > integrand
- sūmō > sūmendus > summand
- multiplicō > multiplicandus > multiplicand
So following the same derivation:
- The word intersect coming from Latin verb intersecō, whose future passive participle is intersecendus, leading to intersecend (or more likely intersecand, to give a hard 'c').
- The word union comes from the Latin verb uniō, whose future passive participle is ūniendus, leading to uniend (or perhaps uniand).
That said, I have never seen the words intersecand or uniend, so using them will probably cause more confusion than it is worth.
...in fact, prior to reading your question, I had never heard the words minuend or subtrahend, either.