The MLP will provide a supplemental sales schedule. it will have all the info you need to compute - ordinary income, short and long term capital gain
it will even tell you the form and even when necessary the line number on the form.
to make it easy split the amounts in proportion to the number of shares held in each account if the sales schedule doesn't split them for you.
technically you don't have to split the reporting. with sales of an MLP only the sales proceeds are reported to the IRS not the adjusted cost basis. as you know the brokers' statements don't reflect the proper cost basis.
