Dee's separation anxiety

Here's my problem:  Deenah goes crazy if I do anything with Dorica, like take her for a walk or even do a lesson with her on the other side of the fence.  Deenah will try to jump fences that she can't clear, push through fencing, get her head stuck, cut or scrape herself ... get the picture?  And she has hurt herself.

She has an ulcer too and I don't want to exacerbate that.  She hyperventilates and really gets worked up.
So I have been very careful and reluctant to do too much with Dorica, thus avoiding upsetting Deenah.

But Dorrie needs exercise and stimulation and she's so keen to do stuff one on one with me.  She seems sad to always be left behind if I walk either Deenah or Siog.  And she's overweight and needs at least 30 minutes of exercise per day.

So tonight by some miracle, she separated herself from the other two and I was able to close Deenah and Siog in the barn paddock.  I'm not at all confident that Dee won't "try" to jump the fence but I struck off with Dorica, leaving an anxious Deenah trotting and braying.

Dorrie and I had a blast, skittering down the dark, steep path through the forest to the marsh and up again.  When we emerged, Deenah must have been in the barn (I always leave her with hay) and didn't see us, so we crept by and through the gate to the road.

Dorica is so well trained - she is soft, alert and attentive.  She softens her neck in a lovely lateral flextion towards me.  Off we went up the road in this fashion to the halfway crest of the hill.  She offered a trot a few times (lovely girl!)

And then turn and back down, the two of us jogging together in tandem.  It was so much fun and I long to do this with her every day - she is so wonderful!  It was late in the evening, near dusk - Deenah was okay when we returned but oh my, it is so hard to do this!  I have witnessed Deenah jumping gates, catching her leg and landing on her back, slicing her nose on fencing, ramming through stock fencing as well as our portable fence panels ... all very dangerous and worrisome.

What can I do so that Dorica is free to work with me, come for walks and have her time out and make sure that Deenah is safe and doesn't cause too much anxiety for Siog either?