It is possible to add conditions for MT_A and MT_B but you need to use just one Message Interface for source. PI 7.0 does not support multiple operations as in PI 7.1 and above. So here is a little POC for you (although I'm using 7.1) 
Please click the images to enlarge.
1. Create a data type (just a simple type category with xsd:string as type) and use it as a message type and then use the message type in a message interface
In this POC, the interface pattern used is Stateless (XI30-Compatible) -> this is the only way to simulate your scenario
2. Create the message types for MT_A_Out, MT_A_Inb, MT_B_Out, MT_B_Inb. Afterwards, create service interfaces for MT_A_Inb and MT_B_Inb.
Service Interfaces
3. Create message mappings/operation mappings for Interface A and Interface B
Operation Mappings:
Note: When activating the operation mapping it will give a warning, but this is okay.
4. Receiver determination is normal receiver determination
5. Interface determination condition is this:
6. Two receiver agreements
7. My testing is done via file to file (in this PoC). Source payload is this:
<?xml version="1.0" encoding="UTF-8"?>
<MT_B_Out>
<XML_Version>2.0</XML_Version>
<Field_88>Field88</Field_88>
<Field_89>Field89</Field_89>
</MT_B_Out>
Hope this helps,
Mark