It really depends on the standard the wifi access points are using, I assume you talking about 802.11g when you say 54Mbit/s. If you have two routers next to each other, depending on the router, will use OFDM to decide which one gets to talk on the shared channel. The total bandwidth cannot exceed that of the channel\spec you are using.
Access points use CSMA/CA which means that if one AP is talking the other one will wait until it is done before transmitting.
This means that if you have two AP's next to each other they cannot transmit more than the bandwidth of the channel (in the case of G its 54Mbit/s). If you add up the total transmitted bandwidth it will probably be even less than the total because the AP's will need some time to figure out who is talking.
It will be up to the Access points implementation of the CSMA/CA algorithm, the amount of data that is being transmitted by both routers and the local interference that determines how much actually gets transmitted. The only thing we know is between the two, you'll never get more than the total bandwidth.