Hoping someone else may have answered by now! There are two notes stacked all the way through this sheet music. It would be better to understand if the top note of each pair had an upward stem, and the bottom a downward stem. Not possible with notes like a semibreve!
As there are two notes, it makes it so that each trombonist has his own line to play. Tr.1 plays top notes, tr.2 bottom. hard to get mixed up usually, and on the occasions where both play unison, that's where the stems issue makes things clearer.
12/8? Well, it means there are 12 'little' beats of a quaver (eighth note) are in each bar (measure) - or their equivalent. Usually gets counted 1 2 3 4 5 6 7 8 9 10 11 12. So, a kind of 4 count, with each count split in to 3.