Hi,
From the online help in Instant:
You can specify three types of bandwidth limits.
Percentage of Airtime: The aggregate amount of airtime that all clients on this Network can use to send/receive data.
Each User: The throughput for any single user on this Network.
Each Radio: How much throughput each radio (some models of AP have multiple radios) is allowed to provide for all clients in aggregate connected to that radio.
So, if you untick the 'per user' checkbox, the bandwidth limit will be per AP; you can increase the bandwidth limit per radio and users on the same radio will share that bandwidth. This might be close to what you want to achieve...