1) Either you would need to apply for a re-entry permit (before you leave Thailand) for your Visa-on-Arrival (VoA) entry. You will need:
- arrival/departure card (TM-6 will be attached to passport page when you enter)
- 1 passport photo 4cm x 6cm, although larger size may be accepted
- application (TM-8, [backup])
- Fee: 1000 baht for single re-entry and 3800 for multiple re-entry permit.
You can get one at the major int'l airport immigration offices. If you will do it at the airport, arrive 3 hrs prior to the flight to give yourself enough time to clear immigration and catch your flight. You can also get it done at a local immigration office, but the airport immigration might be better. Make sure you get the re-entry permit before getting your exit stamp.
Edit: It seems that currently the VoA fee is waived for certain nationals, including Indians. So save the re-entry fee and just do the VoA.
2) ...Or you would have to basically get another VoA when you re-enter Thailand.
Seems like the Thai VoA program is free until October, 2019. If this is the case, just get VoA on both entries.
edit2: don't confuse Visa on Arrival (VoA) with Visa Exempt (VE) schemes. Indian nationals do not qualify for VE schemes as of this writing.
edit3: There apparently is an electronic method to apply for VoA that might speed up your entry process: https://extranet.immigration.go.th/voaonline Screenshot:
If you do use this online VoA method, please comment on how it went after successfully going through immigration.