Series I bonds limit for joint return

Sorry but you are incorrect ... each tax return may have only 1 form 8888 attached to it and the 8888 form has a limit of up to $5K in bonds  even if you are filing a joint return.  

Yeah, it's an Opinion...BUT: 

Bond purchases from the refund get all screwed up if the IRS changes, or delays your tax refund for any reason.

...Just get your refund deposited in your bank account, and set yourself up separately with a Treasury Direct account and buy your I-bonds there.  Yeah, you'd probably have to set two accts up....one  for yourself and another for your spouse......but then you can buy at any time you feel it's appropriate. 

Downside? yeah...Yet Another acct/pwd to keep track of.

There are a lot of financial article writers that got the limit wrong. :(

Yes, Critter-3 is correct, the limit is $5000 per tax return. 

Here is the instruction for this line:

"You may request up to three different savings bond registrations. However, each registration must be a multiple of $50, and the total of lines 4, 5a, and 6a can’t be more than $5,000 (or your refund amount, whichever is smaller)."   - Form 8888 instructions

There are no exceptions for married filing jointly returns.

You can purchase more directly from the treasury ...  however just not thru an income tax return on a  form 8888. 

https://treasurydirect.gov/savings-bonds/buy-a-bond/

Thanks DCW54.  You are right.  TreasuryDirect.gov allows purchase up to $5,000 in paper i bonds for each social security number with their tax return, and this is in addition to $10,000 limit each person has for online purchases.  That should therefore be available in Turbotax, for MFJ filers, imo, but isn't!  Do other tax prep companies allow $5K each for MFJs or is the discrepancy actually between IRS rules and TreasuryDirect.gov rules?