The initial IRS processing of your 2018 tax return does not look beyond your 2018 tax return, so the fact that it may take the IRS several months to process your 2017 amendment is not a concern. If your CPA completes the 2017 amendment before October 15, just use any carryover amounts from the amended 2017 tax return when preparing your 2018 tax return.
Otherwise, if your amendment to the 2017 tax return will not result in any capital loss carryover or any changes to any other carryover amounts, your 2018 tax return will be unaffected by the amendment and there is no reason to delay filing your 2018 tax return. Also, if you will have no balance due with your 2018 tax return, filing late will not result in any penalty or interest since and penalties and interest are based on the balance due.
If filing late would result in penalty or interest, you are probably too close to the extended filing deadline for 2018 tax returns to delay that filing any more. If your amended 2017 tax return will have either a short-term or long-term loss carryover or a change to some other carryover, your choice will be to prepare your 2018 tax return as if you were not going to amend 2017, then amend 2018 to incorporate the changes after amending 2017, or prepare your 2018 tax return using the anticipated carryover amounts that will be shown on the amended 2017 tax return (then amend 2018 if those anticipated amounts end up being incorrect).
Online TurboTax will close for filing 2018 tax returns shortly after October 15, so if you want to file after October 15 you'll need to use the CD/download version of TurboTax.
