Although this situation might end up occurring as the result of unusual circumstances, you would never want to plan for it to occur up front. After all, that would mean you would be asking investors to invest their money while knowing that they can receive better terms if they wait until the next round; this is of dubious ethical character. To fix this situation in your plan, do one or more of the following:

1. Lower the price per share of the earlier round.
2. Raise the price per share of the later round.
3. Accelerate the earlier round.
4. Delay the later round.

Related Questions:

How do I change the price per share for an investment round?

How do I change the date of an investment round?

How do I change the founding date for my company? / How do I change the date of the founders' investment round?