No, it doesn't work that way you can't combine the speeds it's either one or the other. Your compyter will only lock onto one source at a time and as you can already see the modem is the primary and the computer recognizes that and will download from that source , ignoring the cell phone source.
It may be possable to use a server type set up that large compaies use with high priced network cards that can combine download bandwidth but even that is from a single source and I'm sure the high cost would not nake it worth while.