An anti-stacking clause protects the funder's place in line. It usually forbids selling more receivables, taking another advance or granting another lien without the funder's written consent until the purchased amount is paid.
Breaking it is typically an event of default. Depending on the contract, that can make the full remaining balance due at once, add default fees and trigger the personal guarantee. A new funder's UCC filing and the new debit on your statements make a breach easy to spot.
If you need more money while an advance is open, ask your current funder first, or look at consolidation or refinancing that pays the first deal off. This is general information, not legal advice.