Can I apply for a new 1-year Multi-Entry Non-B visa before my current one expires?

Options are there. Time to close commenting.

The way the system is made is: Get a Visa to enter Thailand and then get the 1 year Extension of Stay based on employment from the Thai Immigration Office.

Jumping hoops to avoid the rules, makes things complicated.

Good luck.

Then maybe two options other than in-country extension.

1. Check with the embassy/consulate where the current visa is issued if they can void (or cancel) the current one.

I'm saying this because some consulates mention in their web that only issuing consulate would be able to void issued visa and you need to do it before you apply a new one (in case you need a new one before expiration). (Though I had never read any report like that. you need to check with them.) Then apply a new one there or at another consulate.

2. Keep the current one and do border runs. Get single entry non-Bs (maybe twice) then when you renew your WP get multiple entry Non-B again.

Is there any specific reason you need new Non-B multiple before current one is expired, for example the company name is mentioned on it when you want to change your job, etc?

If you do your last border bounce just before expiration as Robert mentioned, it makes you stay in Thailand up to August, it sounds much reasonable than get new one almost half year earlier if there is not some reason.

make the border run in May, gives 90 days of stay until Aug, time enough to prepare tor the 1 year Extension of Stay at Immigration.

Official you can not have 2 valid visa's in your passport, It is leniency of the Consulate staff to issue a new visa, most of the time within days the old visa will expire. Not months before.

But you can make a border run until the valid until date with this 1 year valid Multiple Entry Non Immigrant B visa and get 90 days of stay as you already are doing

I understand you can't have two valid visas in your passport at once, but I know some consulates may issue them early. But 2 months early? That might be stretching it.