That if any joint tenants, tenants in common or co-partners of unseated lands sold for taxes, have made, or shall hereafter make, within two years after such sale, an offer or legal tender of a proportionate part of the amount of the taxes for which the said lands were sold, and of the costs, together with the additional sum of fifteen per cent on the same, equal to the proportionate part of interest in and of the land so held by him, her or them, to the county treasurer, who is hereby authorized and required to receive and receipt for the same, and pay it over to the purchaser on demand; then and in that case the said joint tenant, tenants in common and co-partners shall be entitled to recover their share and interest in said lands, by due course of law, and shall hold and enjoy the same, with the purchaser at said sale, as tenants in common.