Is the flow rate into both beds the same... or is one greater then the other...

I'm wondering if incorporating a ball valve onto each distribution pipe... and adjusting the flow to each bed might help...

Are we still talking about siphon problems or is this a different problem?
Sounds to me like you are saying the bed further from the fish tank is having trouble draining into the same pipe as the other bed to drain to the fish tank?

Long drain pipes can be tricky and 40 mm might be a bit small for a shared drain line. A fast flow like a siphon can deliver trying to run down a drain that size can cause some interesting things to happen. especially if there are two different beds with siphons sealed onto that same drain line, one bed siphon starting could actually trigger the other bed to siphon too early.

If this is the trouble you are having....... where here are a few options (disregard if your issues are actually something different as you didn't give much details in the post, more details would help us help you.)
1- Put a T in between the beds and run a upright up to about the height of the top of the beds, this will acct as a vent pipe to allow air into the drain line and hopefully keep the one bed from kick starting the siphon on the other bed. Could also help the siphons stop better.
2- use separate drain lines for the two beds. Vent pipe might still be needed if the siphons are still slow to stop.
3- Run a larger (75 or 80 mm I'm not sure the standard pipe sizes there but I use 3 inch drain pipes for multiple beds) drain pipe and just cut holes or use Ts and Elbows for the siphon pipe to drain into with a little air gap to allow for venting.

Could you clarify your actual problem a little more? If your trouble is with the siphon starting/stopping: Make sure you have space between the top of the standpipe and the bell cover once it is sitting on top. From your pictures, it looks like they may be pretty close. Too much space is better than no space.

having trouble with my siphon working all the time.