In such case each device needs to be able to cooperate with the fact that another device is currently talking and the bus is therefore busy.
This translates into:
a) Being able to follow arbitration logic. If two devices start to communicate at the same time the one writing more zeros to the bus (or the slower device) wins the arbitration and the other device immediately discontinues any operation on the bus.
b) Bus busy detection. Each device must detect an ongoing bus communication and must not interrupt it. This is achieved by recognizing traffic and waiting for a stop condition to appear before starting to talk on the bus.
If you plan to use a multimaster device on a bus it is essential that all masters are multimasters. A single-master is simply a device, which does not understand the above mechanisms. If a singlemaster and a multimaster are connected, the singlemaster may well interrupt the multimaster causing unpredictable results.