I have a 89 BMW which seems to be double locked due to Battery being Dead. I was able to get in the car from trunk and open the back door. But I am not able to use a Key to open either the driver or passenger side doors. Battery is charged now.

If the battery is under the back seat like some models, charge battery.

The outside door locks/trunk lock will most likely not work due to the key not being able to be inserted/turned in the lock cylinder due to lack of use. Spray WD40 into door lock cylinder using supplied straw and insert key until it goes all the way in using sawing motion. If lock cylinder still will not turn continue sawing motion with key while holding gentle turning pressure on the key. This will usually free up any sticky/seized lock cylinder wafers and allow the cylinder to turn

you can call a key smith which will come out and get into your trunk but also the key smith could change the lock to the trunk so the same key that opens the doors also opens the trunk.

open trunk, disconnect rear trunk light. use jump box and connect wires to pos neg on jump box. this will give u enough juice to open doors, I have done it twice. easy to do

in the trunk. you will need to get under the hood if the trunk wont open to charge the battery. There is a post on top of the intake manifild

Why dont you call loc smith to unloc the doors
You could take flat screw driver to try push the latch side ways where it latches

if the doors and trunk are vacuumed operated, make sure all the doors are unlocked..this might make it easier to open the trunk..or get the car running and then try to open the trunk with the car running..turn key all the way to the left and push slowly in..I think it is vacuum operated like the mercedes

break the lock and replace or try removing the back seat to gain entrance to the trunk and retreve your key

Connect jumper cables to the starter connection which should be visible under the car, hot to the bolted battery cable connection, negative to any ground. Connect the other end to a battery after you have crawled out from under the car.  Also consider; my BMW had the battery in the trunk, which should open with a key.

Try to hook up a battery charger to the battery post of the starter to CHARGE the battery enough to open the doors.
Jim