I am wiring a hot tub. The spa subpanel I purchased (Eaton) came with a 50 amp duplex GFCI breaker which is what many spas use. I bought a used Hot Springs Spa and found instead of a single 240 volt feed their wiring diagram shows both a 20 amp split phase 120 V configuration, utilizing a neutral as well as a 30 amp GFCI which feeds the heater, no neutral just ground. I replaced the 50 amp GFCI with a 30 amp GFCI and have no problems. The potential problem comes with the inability to install an addition 20 amp duplex GFCI breaker in the panel. I can and have connected single 20 amp GFCI breakers where L1 and neutral feed one circuit and L2 and a neutral feed the second circuit. There is only one neutral connection to the spa controller. A duplex ckt bkr has one neutral pigtail and one neutral load connector. My configuration has two each pigtails and neutral load connectors. The only way I see to wire this is to jumper the two neutral load connections together with the neutral wire feeding the spa. I also have connected both pigtails to the subpanel neutral bus. My question is whether there is something electrically different about a duplex breaker and two single breakers (both GFCI). I notice when I press the test button on either breaker they both trip, so I think it is functioning properly. Each 120 V circuit feeds two different loads; circ pump and a jet pump, so at least in spirit I don't think they require a common trip handle. I have already spent too much money on breakers and if this configuration will work I would like to use it, otherwise I will have to buy a panel that will let me use two tandem GFCI breakers and of course a tandem GFCI breaker at at additional cost of nearly $200. I would appreciate your inputs. If you have experience with the Hot Spring permanently wired 240 V spas, please mention that in your reply.
You cannot combine neutrals from two GFCI breakers; it just won't work. A GFCI functions by comparing current flow between hot and neutral (for a 120 volt breaker) or hot 1 and hot 2 (for a 240 volt duplex breaker). Because you've combined the two neutrals, the current between the hot and neutral won't balance.
I have no information on modifying your spa, but if you cannot change it, you'll need a different panel and two duplex GFCI breakers.
The answer is NO you can not use 2 separate GFCI’s to create 240v. You can not parallel breakers except in supervised or listed conditions
The hot springs spas I have wired require a 30 amp 240v and a 20 amp 120v feed with a ground on the 240 that you don’t show all on a 50 amp feeder the actual wiring diagram was on the inside cover of the controller not the block diagram you show on the right or the incomplete drawing on the left. If the used spa panel you purchased can not handle a double pole GFCI and a single pole GFCI you have wasted your money.
I just pulled up the MFG website to make sure what I said was still accurate as it has been a couple of years. They still wire their spas exactly the same way. 30 amp 10 awg hot hot ground, 20 amp 120v hot neutral 1 model had a ground but that was for 230v control make sure of the voltage and verify the proper jumpers on the terminal block I had forgotten about that.
You need a 30 amp 240v GFCI and a 20a 120v GFCI to connect to MFG specifications. It looks like the wiring diagrams start on page 31 of the 2019 model earlier models are a few pages later.
Just a note, used electrical equipment is rarely a good deal in my experience. Code changes every 3 years and in some cases you could be just purchasing a new anchor for a boat or a device that works today but fails next week because of age.
Make sure to use the schematic inside your controller even if the same brand as your hot tub the wiring can be different than online or older sub panels sold by the same mfg.