Twin monitor question?

A splitter only duplicates the screen. 2 diff screens require separate video connections without a splitter. You will have a hard time finding an old dual monitor video card that works on the under powered Optiplex. Why are 2 monitors even needed? 

Can you recheck which model # you have?  An Optiplex 970 doesn't exist.

Are you sure VGA is your only output option?  VGA being the only option on an Optiplex goes pretty far back, including the DT's.

Whatever Optiplex you have, I'm sure we can get something working.  They are not underpowered for 2 monitors.

Yes it is  970. but maybe discontinued, as the one I have is a refurbished one from Amazon UK

It has a Display Port as well as a VGA one, so I ordered this to adapt to the VGA cable I have.....

https://www.amazon.co.uk/DisplayPort-Adapter-Display-Converter-Gold-Plated/dp/B017Q8OCSY/ref=sr_1_1_sspa?dchild=1&keywords=displayport%2Bto%2Bvga%2Badapter&qid=1591192666&sr=8-1-spons&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzRVlMTlM2QVlIRzU3JmVuY3J5cHRlZElkPUEwNjk2OTMzMkFEQTlIU1pBTTRJRSZlbmNyeXB0ZWRBZElkPUEwMTU4NzA4RjhPUlpPQlQ1TVExJndpZGdldE5hbWU9c3BfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ&th=1

Thank you all for your help.

Glad you found an adapter for it.   Sorry for not believing right away that it's a 970.  Dell made sooo many Optiplex's, it's not too unusual for one to come up I never heard of before.  I researched before making the post, "Optiplex 970" in Google and nothing - reverts mostly to the 790.  Today, I used "Dell Optiplex 970" in Google and it coughed up one.  It must be pretty rare.  I couldn't pull up a manual or tech guide, nothing in ebay and Amazon (U.S.).  The one I did pull up looked relative to the 790 and 990.

Ha, It's me that should be apologising to you.

It is a 790, not 970

I didn't think I was numerically dyslexic.

That's ok.  Funny how an advertised "970" still came up on Google.  In the photo, the model # was wiped out.  Some sellers do that to keep reusing stock photos.

2nd monitor could use usb.

https://www.amazon.com/Adapter-Multi-Display-Converter-Projector-Chromebook/dp/B07RTB9W14

Sorry for the late reply.

I bought that USB adaptor SpeedStep but couldn't get it working.

A GT 710 will do a MUCH better job at powering both monitors.  It has a TDP of only 20 watts, so the power supply can more than handle it.  And it's only $36 US.  You can even do light esports gaming on a 710.

https://www.amazon.com/Computer-GT-710-1GD3H-LPV1/dp/B01DOFD3RY/ref=sr_1_1?dchild=1&gclid=CjwKCAjwrcH3BRApEiwAxjdPTdu-Sr2KKqUOHsVKm-1sT6DAsGW_GYVXjiQNGUGfmicvhdLSEdj62RoCuMYQAvD_BwE&hvadid=324849259425&hvdev=c&hvlocphy=1001970&hvnetw=g&hvqmt=b&hvrand=17940094171760205245&hvtargid=kwd-296019468610&hydadcr=24950_10283306&keywords=msi%2Bgt%2B710%2B2gb&qid=1592840288&sr=8-1&tag=googcana-20&th=1

USB video adapters require drivers to work. They are not plug and play.

OK, so I had to add a low profile Graphics card.

But since then The curser/arrow has the little blue circle continuously flashing with it to the right of the curser, a bit off-putting.

Is there a cure for this, please? 

What GPU did you add?  What size is your 790?  Components vary in between sizes.

Optiplex 790 Tech Guide