[nemesis1637]

'That situation is simply not present in Spain because they issue residence permits very quickly, before your 90-days visa free stay expires.'

This is a common misconception. For almost all residency permits, you must apply from the United States. You are not allowed to apply while in Spain. There are exceptions, but not many. Though, with the new digital nomad visa which did not exist when I moved, it is possible to apply while in Spain. But, it most likely will not be issued within 90 days. I have many friends that have done so and are forced to live illegally for a period before it gets approved. I've never seen a single residency permit get issued within 90 days (other than student visas). The Govt. is EXTREMELY slow. Much more so than the U.S. even. Even our renewals have taken much more than 90 days each time. The easiest option that some people do when in a bind is to enroll in Spanish classes and get a student visa--that one can be applied for in country and is relatively 'fast' (in Spanish terms).

'to firstly get a residence permit in Spain, then go to Portugal and apply there, and live in either country until Portuguese one comes out.'

For most residency permits (in Spain) you have to physically be in the country for more than 183 days (half of the year). So this is not possible (unless you want to switch back and forth). Your renewal would not be approved.