Nice to See You Again
 

When I haven’t seen my neighbor for a long time, I usually say:

“Me alegro de verte de nuevo” or “Me alegro de volver a verte”

I’m unsure, however, what to say if I am talking to two male neighbors. Is it “verles” or” verlos”?

What do I say if it is two female neighbors? Is it “verles” or “verlas”?

Any input and/or explanation is appreciated.

It's correct to use 'verlos', since the two male neighbors are the direct object, but 'verles' is allowed, especially in Spain.
If your neighbors are females, use 'verlas'.

If the neighbors are one male and one female, I assume I would still use "verlos"; am I correct?

Yes, the masculine form in Spanish becomes neutral when there are women and men in the plural.

Side note: In Mexico, when you haven't seen for a few days someone you see often (neighbours, friends, coworkers, etc.) you may say informally: "Gusto en verte/verlos/verlas."

Rusty:
Thank you for the explanation. I was leaning toward using "los" and "las," but I was unsure and wanted to find out for sure.

Angelica:
I have heard "Gusto en verte/verlos/verlas," but only after the conversation had concluded and my neighbor was departing. That gave me a feeling that he was saying, "It WAS nice seeing you." Can it also be used at the beginning of the conversation to people you have not seen for a while? For example:

Hola, Julio. Hace mucho tiempo que no te veía. Gusto en verte/Gusto en verte de nuevo. ¿Cómo te ha ido?

Oh, you're right. That's for saying goodbye.

If you're saying hello, you may say: "(Hola) qué gusto verte. Hace mucho que no te veía". :)

Thanks, Angelica, for the clarification.

In Spain there's many verbs where they will use "les" instead of "los " when the objects are people, while in other countries they will only use "los" in those situations. So basically both forms are correct (you can't say the Spaniards are speaking their own language incorrectly.)