In Stage 6-2B, grab the 1-up, backtrack to 6-2A, then return to 6-2B. The 1-up will have reappeared, along with all other power-ups in the room. You can repeat this process until the timer runs out.

In Stage 7-2D, grab the 1-up, backtrack to 7-2C, then return to 7-2D. The 1-up will have reappeared, including all other power-ups in the room. Again, this can be done repeatedly until the timer runs out.