If you're not opposed to opening new accounts, then yes apping for a new Platinum (or Reserve if it makes financial sense) when it has an elevated 70k SkyMiles offer is the better option. If it were a matter of just doing product changes, I'd evaluate two things - do you plan to fly together at least twice a year, and do you either of you have any intention of going for Medallion status? If the answer to either of those is yes, it may be a better option to upgrade both cards to Platinum (or Reserve, again if it makes financial sense). Keep in mind that if you downgraded your card to Blue, if you flew solo you would not have the free bag or other benefits available to Delta cardholders (even things like buying tickets with Miles + Cash). Those companion certificates can more than pay for the cards, as long as you use them.
FWIW, I got the Platinum in October and DW got the Gold in October. We are both planning to upgrade to Delta Reserve in January (so that any MQM bonus is applied in 2019 for 2020 Medallion status); kicking myself for not getting DW a Platinum instead of Gold since we almost always travel as a family of 4 and that first companion certificate is coming in a few months.
Now you got me thinking of doing the same thing with my Delta card. I was debating on downgrading or going for the Platinum/Reserve card.
@sdsoccerdad wrote:Now you got me thinking of doing the same thing with my Delta card. I was debating on downgrading or going for the Platinum/Reserve card.
I need to figure out a way to monetize this with Amex. 
@Anonymous wrote:My wife and I travel quarterly on Delta, generally CVG-TPA, although they fly into SRQ now too. We ALWAYS travel together, and at least once a year take either parents/in-laws or college aged kids with us (on my dime). This isn't quite as cut and dried as I might have expected.
CVG-TPA would likely be break-even with the companion ticket in Main Cabin for Platinum, and First for Reserve. CVG-SRQ you should be ahead about $50 on the AF with either card. If you always travel together, then it certainly makes sense for at least one of you to have Gold/Plat/Reserve for the bags (especially when you start adding in your companions since up to 9 of you get the free bag). Not cut and dry at all if you actually travel on Delta. For solo travelers, it can be a little more black and white, especially if they already have other premium cards.
@K-in-Boston wrote:
@sdsoccerdad wrote:Now you got me thinking of doing the same thing with my Delta card. I was debating on downgrading or going for the Platinum/Reserve card.
I need to figure out a way to monetize this with Amex.
Absolutely! If my wife and I both got it, it would make sense since we are a family of four and tend to use Delta as much as we can. From what I have learned the past two years is that I should not apply for anymore cards unless there is some kind of positive value to be expected from it.
i agree with the group here as well. I downgraded my card to the blue then apped fresh for the one i wanted for the bonus miles. Good luck 