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Just to add on to what jacob said above, if one or both of you ever did plan to establish, neither would be able to since you might be within 3000 blocks of their lock chest. Unless of course you both agree to let the other establish that close. Safest bet would be to be at minimum 3k away so that at least the option to establish remains without needing anyone's approval.

I really figured that there was a rule and I just wasn't finding it. Glad to know I wasn't incapable of reading the forums.

Alot of players build right within that 5k block area near spawn and I was seriously starting to question where building should be done there, not 'can' because .... well, those blocks are already on top of each other. Some day those Frontier spawn areas are gonna get to recover from the nuclear bomb that has decimated some of them. It would be too much to think that repairs could be made. I know that work right now is continuing in pursuit of 1.13/1.14
Still.... those areas will never be beautiful.

I agree. Would be nice to hit a spawn area and just see the natural untouched world generation. Unfortunately this issue would never be resolved. If they made it so nothing can be built with 5000 blocks, the areas starting at 5001 blocks would look that way too.

I believe not so much a rule but advise was not to build within the yellow bounding box around spawn as that area can be reset like wastelands, and if you want to establish a base it must be out a least 8k blocks from spawn and 5k blocks from another players unless otherwise agreed.

You can however build where you like more or less as long as you understand the above.

Happy building

advise not build within the yellow boundary line??
I think you may have mis-stated that. You are not allowed to build within the boundary of spawn.

I was really looking for the rule about not building on top of someone else's build, even if they are derelict. Being told that there is a 100+ blocks that is customary in the Frontier, but there might not be a hard and fast rule.