How do I set aside money for both taxes and retirement at the same time?

Treat both as fixed claims on every payment, taken off the top before you decide what's spendable. A clean approach is to assign percentages to each goal: when money arrives, move roughly 25–30% to a tax account and another chosen percentage — say 10–20% — to a retirement reserve, leaving the rest for owner's pay and expenses. Using separate accounts keeps the money from blurring together and being accidentally spent. Order matters: taxes are non-negotiable, so fund that first, then retirement, since contributions to a Solo 401(k) or SEP can also lower your taxable income and ease the tax bill. The Profit First style of multiple accounts makes this automatic. In strong months, push extra toward retirement; never raid the tax bucket to do it. Recalculate your percentages as income grows so both goals scale together rather than competing.