I have a double entry visa. The first visa is almost expired. When I do a visa run, is it up to me to get a 30 day entry stamp or am I obliged to use the second visa, which I preferably would like to keep a little bit longer ?

Usually they'll take the second visa, Them's the rules. You can try. No guarantees

Ozzie friends of mine do the following with a double entry six months validity tourist visa:

Obtain 60 day stamp on first entry.

At the end of 60 days obtain a 30 day extension from immigration (Bht 1,900.00)

At the end of 90 days, leave the country and return obtaining a further 60 day stamp based on their visa.

At the end of 150 days obtain a 30 day extension from immigration.

It appears to w*rk for them.

It'll work for almost anyone