IRS form 7203 Instructions, page 2, states:
Item C
When a shareholder has a different basis in different blocks of stock, pass-through items are generally allocated pro rata to all shares, regardless of their different bases. If there is a partial stock sale or partial redemption, you may file more than one Form 7203 and provide a description of what period the form covers. See Regulations section 1.1367-1(b)(2) and (c)(3) for details