How to apply weighted links for 2 Mass points to a tube end surface?

In the above screenshot, there is a Pressure Relief Valve (PRV) and an Adapter installed at the end of tube (pointing downward). When I applied two Weight links (as shown in the 1sigma stress plot at the right side), the max stress is 100+ times higher than the one shown in the plot at the left side, in which only one weight link is applied. What is the problem to two links? Do I have to combine the two masses into one mass point in order to simulate both mass contribution from Adapter and PRV?
Thanks.
Also, is there a good reading about "weighted link" (and "rigid link")?

