Very tangled-up info...

1.  Clients sold the business. Sold 720,000.00 cash payout 510,000.00 split between two partners. Installment sale for the remaining 210,000.00.

2.  Each partner received a 1099-S for the full 720,000.00 and each partner received an installment sale agreement for the 210,000.00.

2. is contradictory to 1.   How about requesting a correction for the 1099-S and "Installment sale agreement" issuer/preparer???  

As @abctax55  pointed out, was the 1099-S even appropriate?

If the Installment sale agreement was prepared in the name of the TWO partners as the selling party, and each received a copy, it might not be incorrect.  If the copy received by each partner was in the name of the partner, how about the payment amounts stated on the agreement.  Was it based on the total ($210,000)?  If so, did the buyer pay  (or has been paying) the doubled-up amounts?

3.  1099S issued under SSN, for the full amount for each partner.

Did you review the sale/purchase agreement?  Who exactly was the selling party - the partnership or the partners? 

4.  They sold the business, a/P, A/r, and inventory.

Bear in mind that inventory is not eligible for the installment method.  (IRS Pub 537: The sale of inventories of personal property can’t be reported on the installment method. All gain or loss on their sale must be reported in the year of sale, even if you receive payment in later years.)

Neither is the partnership interest that was attributable to unrealized receivables.  (For example, see Mingo, T.C. Memo 2013-149)

Who buys A/P?  I have some for sale.

5.  It is on the K-1's line 20P.

What was the "it" that was on the K-1 Line 20P.  How much?  And why?  Who prepared the Form 1065?  Has the partnership return been filed?  

Key general point: As in any other tax area, make sure to have all the necessary information in reporting a transaction.