You have not given us your profiles. Just a start, an end, and a depth somewhere in between. Unless you are starting at the top of a wall, these will be far from square profiles.
But the point is his only planning tools appear to be tables that he apparently doesn't have or perhaps even know how to use....so the only safe assumption for him is a square profile, so the only safe answer for him is to shorten the second dive. If he uses the PADI RDP, the first dive (50 ft) puts him in PG O, which after a 75 min SI is PG E. That's 13 mins of RNT at 75 (80) ft, so his remaining BT is just 13 mins. He could do a 13 min dive, but not a 45 min dive.
If he has a computer, the game changes....but only a little. Even if his first dive (being multi-level plus a 75 min SI) gives him
zero residual nitrogen, he still can't go to 75 ft for 45 mins on the second dive. His second dive must also be multilevel, and most likely also be considerably less than 45 mins.
Added: If the eRDPML is used, instead of the RDP, you can work with 5 ft intervals instead of 10 ft. On that basis, the first dive ends in PG M, reduces to C after 75 mins SI, and allows 24 mins BT on the second dive to 75 ft.
If the dives are treated as multi-level, using the eRDPML, and the first dive is 45 mins to 45 ft followed by a 75 min SI, the second dive is possible as a multi-level dive, for example: 15 mins at 75 ft followed by 30 mins at 45 ft, plus a normal safety stop.
This kind of dive planning is not possible with the usual tables, nor with most computers, but the eRDPML makes it easy.
Advice to the OP: do NOT use my dive planning for YOUR dive. do your own dive planning.
ADDED LATER:
The OP should NOT do these dives, even with the multi-level aspects outlined above. With the usual AL80 tank, he MAY not have enough gas to do the first dive, and he WILL NOT have enough gas to do the second dive, not with a single AL80. He asked questions about his nitrogen management. His bigger concern is his air supply.