Radio model: Siemens super A 7s
The Electrolytic cap is a combo with 2 caps of 50 mfd.

Can I use a 100 mfd Electrolytic cap, join the 2 negatives and ground; and connect the positives to the E.cap?

M KHan

No

The set requires two separate caps to filter properly. The two positives of the original caps will be connected to different points in the original wiring. You will need two caps of 50 mfd. or slightly larger capacity, at a voltage rating suitable for your radio. Don't have the schematic but if it's a transformer power supply radio caps rated for 450 volts are probably correct.

...the two 50µF sections are connected to two different places, so you need to use two separate 50µF (you can use 47µF; those will be far more readily available today) caps. You are partially correct: connect the two negatives together and connect them to the chassis, but you will need to disconnect the wires from the two positive terminals of the original 'can' and connect each wire to the + of one of the new caps (installing a terminal strip under the chassis will help, just make sure you're not using a lug that is grounded when the strip is mounted). It is important that the old cap be disconnected (don't simply put the new caps across the old one) even if you want to leave the can in place to keep the original appearance...
...the original part is probably marked something like 50+50µF 350/385V; modern replacement caps will typically be rated at 450V, which is fine (always use equal or higher voltage rating than the original)...

...oh...someone beat me to it...

no but I think you can clampet in a vise let jethro saw it in half with a hack saw glue some connectors back on for 2 X 50s. either way it wont work