No. I guess what your trying to find out is if you had say 2 that had connection speeds of 56k would that give you 112k. The answer to that is no. It may be possible (but unlikely) to have 2 internet connections but that means 2 sets of bills and hence twice the cost.
I would guess the answer is yes, when connecting to a local area network - it won't make any difference when connecting to the internet...
If speed is an issue, you might want to consider some of the high speed wireless networking products out there as 54 and 108Mbps are available for sure... (try http://www.netgear.com/products/wireless/108Mbps.a sp
for example)
No point going faster than 108Mbs unless you have gigabit ethernet though! ;)