Welcome. My name is ***** ***** would be glad to assist.
The main cable supply connects to the utility and then to the fuse connection, then another cable on to the meter, then to the house panel.
If the panel has a 100 amp main breaker, then it is at its maximum already for supply. The rating on a panel is the maximum amperage allowed to the panel, not necessarily what you actually have in place.
The amount you have in place is dictated by the fuse size and/or main breaker in the panel.
Unless the fuses are 100 amps, The cable to the fuse connection is larger than the cable after the fuse connection.
The fuses protect the wiring to the meter and on to the panel.
This would be why they can connect prior to the fuses and add another set of fuses for the second meter because the cable prior is rated for a higher amperage.
In order to increase the fuses to the existing panel now, all the wiring after the fuses would require changing in order to have the panel at its rated maximum of 100 amps. Also, there would have to be enough room in the panel for all the addition
Hope the explanation is understandable, Let me know if any clarification is needed,