Clearance process for different agencies

I’m looking at various internships from multiple agencies. Can someone provide insight about the different agencies and which does their own background process and which uses the same process? I understand that DIA, CIA, FBI, NSA, NGA (and maybe others) all use their own process and a new SF86 would have to be completed for each CJO from those agencies (and all would be TS/SCI anyway). What about Department of State or Secret Service, when only a Secret is needed? Do they use the same process (via OPM?)

The reason I’m asking is that my understanding is a TS/SCI for a three letter agency will likely take at least a year (if not longer). While waiting for that clearance and final intern job offer (for two summers from now), would it be possible to accept a CJO from the DoS and/or Secret Service and fill out a new SF86 for a Secret clearance? Presumably, the Secret clearance will take less time, and I’m wondering if I could do a semester or summer internship at DoS or Secret Service while waiting for a TS/SCI to clear for another agency. Will the Secret clearance process from DoS and/or USSS delay the TS/SCI three letter agency process or visa versa?

Can one have a TS/SCI clearance process underway at a three letter agency and a Secret clearance process underway at DoS and Secret Service, at the same time?

Of note, internships I’m considering are for different periods, including two different summers and two different part-time internships while in school, so theoretically (aside from the clearance process), it seems possible to do all the internships and not have to choose between them.

DOS does there own…currently been waiting forever for a DOS secert.

Try at least 3 years for TS/SCI, if not longer. Half sarcastic half realistic