Can I pay for plus to access the past two years of tax return without filing with them this current year

We can tell you how to pay for PLUS, but need more info.    Please answer these questions:

1)  Are you asking how to pay for PLUS without filing a 2017 TurboTax return?    
2)  Have you started a 2017 return?  If not, I'll tell you how to start a Free Edition return, then immediately upgrade it to PLUS and pay the 29.99 by credit/debit card.  You will not have to complete the return.
3)  Have you located the prior returns, and are they in the SAME account that you are using for 2017?  And when you try to view/download/print, do they say you have to pay?

Be aware there's a free option if a tax return transcript would suit your purpose.  You may be able to immediately download a prior-year return transcripts at this IRS website at the link below.  If you don't qualify to open a free IRS account there, you can order one to come by postal mail, which takes 5-10 calendar days.
<a rel="nofollow" target="_blank" href="http://www.irs.gov/Individuals/Get-Transcript">http://www.irs.gov/Individuals/Get-Transcript</a>

Yes can I pay for plus without doing a 2017 return

Yes, you can.  But you have to start the 2017 Free Edition return in order to upgrade to PLUS and pay.  And you must be in the same account as the prior returns.  See the full answer with the steps below.

You can start a 2017 online Free Edition return in the same account as the prior returns, then upgrade to PLUS, and pay 29.99.  You only have to start the 2017 return long enough to upgrade and pay for it.  You don't actually have to complete the 2017 return.

NOTE:  Before paying, make sure you are seeing evidence of the prior returns in the "Your Returns & Documents" section of that same account.  Then here's how to proceed:

• Log in and start a 2017 Free Edition return in the same account as the prior returns.  Or if you have already started one, then continue it ("Take me to my return.")
• Once you are past the blue-green screen, look in the left menu column, and there should be an Upgrade link
• Click on Upgrade and choose PLUS (29.99).    NOTE:  If the PLUS option is grayed out, that means you are already in PLUS.  Do not upgrade any farther.
• Once you are upgraded to PLUS, you need to pay for it.
• While the return is still open, click in the left menu column on Tax Tools, then choose Print Center.
• Then choose "Print/save/preview this year's return."  (Yes, you say "this year's" since you are paying for the 2017 return to unlock the prior returns.) You won't actually have to print it.
  
• On the next screen choose to "Pay now".
• The next screen should be an Order Summary.  Review that, and if all is OK, click Continue.
• It should then prompt you for payment by credit or debit card.  The screen only says "credit card", but it will take a debit card.
  

NOTE:  If it should offer you the option to "pay later" out of the Federal refund, that can't be used for this unlocking purpose.  You will need to pay now by credit/debit card.