yes, that second $100K is going to cost you. but how much we can't say. If this is rental property first, you must recapture as section 1250 gain all the depreciation taken. this can be taxed at up to 25%. next you'll have $300K of taxable income which will subject you to a 3.8% net investment income tax on about $50,000 or an about $1900 on top of the regular income taxes. it's even worse if $100K is short-term. long-term gain, this does not include section 1250 recapture, gets taxed a preferred rate possibly only 15% while short-term is tax just like other household income.
you can use this link (2022 but should give you a reasonable estimate of 2023 taxes) to get an estimate and do various scenarios to see the effect. state taxes are not included.
https://turbotax.intuit.com/tax-tools/calculators/taxcaster
it's probably more tax efficient to sell in different years but no one knows what 2024 will be like. so, there is economic risk in delaying the second sale.
