The first Double Dragon on NES was notorious for removing the arcade version's biggest selling point, its co-op gameplay, but the developers managed to include it in the sequel. Double Dragon II has three different difficulty levels, although you'll have to beat the highest of the three to see the "true" ending. Aside from the Nintendo Switch's version ability to rewind or create save points at any time, the original version had a stage select code, which is a boon.
As with many games of the era, public opinion of Double Dragon II seems to vary based on your level of nostalgia. NintendoLife's review of the Wii release, for example, scored an 8 out of 10, whereas its review of the Wii U release earned a measly 4 out of 10. I agree with the latter, and the reviewer complains about the quirky control scheme (one button is used to attack to the right and the other to the left) and the poorly designed and frustrating-even-with-save-points platforming mechanics. There's some variety in the fight moves (including a hurricane kick executed by pressing both buttons at once), but as with most beat 'em ups the enemies are repetitive and the action gets old pretty quickly. The game is pretty short, though, especially with the modern convenience of save states. The easiest difficulty only includes the first three levels, and so can be completed in around twenty minutes, and even the hardest difficulty (which includes one more stage and also ups the enemies' HP and attack speed) can be completed in less than an hour and a half with save states.
Overall the NES version of Double Dragon II is mindless co-op fun but doesn't particularly stand out amongst beat 'em ups, aside from fixing the egregious omission of co-op of its predecessor. It's nice to have on Nintendo Switch Online and worth a playthrough with a buddy or young kids, but otherwise not worth spending much time on.