If I've held all of the shares held in Robinhood for more than a year
but I've held the same stock for less than a year in Webull how would
that work out for taxes?
If you sell the shares you own in Robinhood, you'll have a long-term capital gain/loss from sale of those shares. If you sell the shares you own in Webull you'll have a short-term capital gain/loss from sale of those shares. If you sold all shares from both accounts nothing above changes, you'd have both long and short-term gains/losses to report.
If all the shares were in one account, and you sold only the number that you've held for more than a year, you'd have a long-term capital gain/loss from sale of those shares. That's because by default brokers will sell your oldest shares first (FIFO - first in, first out).
With some brokers you can elect which shares to sell if you don't want them to sell your oldest shares. Having multiple accounts doesn't affect your tax rate, it does make it easier to pick which shares to sell in a case like yours where the brokers don't support deviating from FIFO.
Be mindful of wash-sale rules when using multiple accounts, your brokers don't know what you are doing in other accounts so it will be on you to correctly report everything. Here's a decent introduction to wash sale rules.