433mhz wireless problems

So - two-way isn't possible?

Apparently not with one pair but you could always use two pairs.
Not sure if interference becomes a problem. If you cant change the frequency then you'd need to make a protocol to stop collisions.

I have another pair and will try out bi-directional at some point.

Certainly it becomes more complex, but I thought add some control data

Source ID - which device is sending
Destination ID - which device should not ignore the packet
Checksum - something simple.

Maybe add
Packet ID - so destination can acknowledge packet with non-zero ID
Group ID - enables multicast

For Collision detection/avoidance.
This looks interesting. Exponential backoff - Wikipedia

Hmm, it's probably all a bit much for me, very interesting though.
While I was googling I found.

http://www.faqs.org/rfcs/rfc1162.html