Just my two cents...
When taking a bird to the vet there are many considerations. If you can afford to do all the tests then you should do them if not then you need to make some decisions. The first thing you need to decide is what vet to take them to. You need to find one your can trust and I personally would pick the vet who has the most skill and resources, that would be your board certified avian vet. If they are not board certified you might try and find out how much time they spend treating birds compared to other animals. 10%, 20%, 50% more. There is no right number but if you are comparing vets go with the one with more experience. If cost is a concern you still want to go to the better vet, their experience will more than pay for the higher cost and they may be able to help you sooner and better and will not only be better for the bird but will be better all around.
Now that you have a vet you trust, the next thing you need to do is REALLY TRUST THEM. Do exactly what they say, it is good to ask questions and decide together on a plan but then you need to follow through. You can not change course in the middle or skip things. Diagnosing an illness often takes time and has many twists and turns. You have to commit to searching out the solutions and follow through, if you give up in the middle you will have wasted all the previous money and work. If you skip a recommended test then you are making it difficult or impossible for the vet to do his job. Once you say no to a requested test you can not blame the vet for not knowing what is wrong. If you don't do what they suggest then it really limits what they can do to help your bird.
I always listen to the vet and ask them to explain each test, what they will find out by doing it and how they will treat that particular problem if they find it. This helps me in discussing the best way to proceed.
In the case that there are two birds with the same symptoms I would take them both in to the vet and discuss the situation with them. This way they can decide if they want to address one at a time or both together. I have had two birds with the same symptoms before where the vet did a more extensive example on one than the other and recommended treatment for both.
In my case it was an environmental issue that needed to be address while also offering supportive care so that there was no need for extensive lab tests. Your case is probably different. I had to make multiple visits over several weeks, and consult with the vet and vet techs daily to get the situation sorted out. Sometimes, the vets need to see what happens with a treatment to move forwards so again it is important to follow through and communicate with your vet.
Personally, I use different vets in the area depending on the situation. The one closest to my home is good if I already have a pretty good idea what is wrong (because it is very simple), they are close and can handle simple things. Another is much farther away but well equipped and very competent. Another is even farther away and is board certified. She has limited hours and is not always available but is worth the drive and wait if you really need a good vet or surgeon. I would still go there first if she was available.
These are just a few things that I have learned over the years. I hope it helps.
Best wishes to you and your birds.