Only get a 2nd bird if you personally want one and have the time, money and energy to do everything with them separately in the event that they do not get along. If your old bird hates the new bird, you will be stuck with it (and vice-versa), so make sure you consider all of the potential complications before diving in. It can work, but since there is no way to be certain that it will, it's important that you only buy a bird based on your own desires (not a desire to help your other bird).
Here are some possible outcomes:
1.They get along well and life is grand.
2. They appear to get along but only for short periods of supervised time.
3. They do not get along (meaning you can never have them out at the same time and interaction may have to be done away from the other bird due to extreme aggression, jealousy, screaming etc).
4. They get along so well that they start behaving sexually and laying eggs/getting hormonal (different species can still be the source of hormonal behaviors in others--as evidenced by your statement that you even saw a budgie trying to feed a tiel). Hormonal birds can experience behavior and health problems.
5. They get along so well that they become jealous of the other bird's interactions with you (becoming defensive when you are around).
6. They get along so well that it is harder for you to interact/bond with one or both of them.
*Birds that do get along at one stage may not get along forever* . With 2 birds, you will always have to be hyper-vigilant. I know there are members with multiple birds, but there are also members whose other birds have lost eyes and toes to other birds in their flocks. It is kind of a coin-toss.
Also, just because you have 2 birds of the same gender and a different species, it does not mean that there will be no sexual behavior.
No matter what, you are going to need a separate cage for any new bird because you will have to quarantine initially.
I wouldn't house them in the same cage anyway-- even post-quarantine.