# Use Derived tag write expression to write back to two other tags

**URL:** <https://forum.inductiveautomation.com/t/use-derived-tag-write-expression-to-write-back-to-two-other-tags/62584>\
**Category:** Ignition\
**Created:** [August 2, 2022, 12:27pm UTC](https://forum.inductiveautomation.com/t/use-derived-tag-write-expression-to-write-back-to-two-other-tags/62584 "2022-08-02T12:27:29Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Arnaud\_Declerck](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/arnaud_declerck/32/44872_2.png) [@Arnaud\_Declerck](https://forum.inductiveautomation.com/u/Arnaud_Declerck)\
**Post date:** [August 2, 2022, 12:27pm UTC](https://forum.inductiveautomation.com/t/use-derived-tag-write-expression-to-write-back-to-two-other-tags/62584/1 "2022-08-02T12:27:29Z")

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Hi,

I'm using a derived tag to read and concatenate two tags. This is working but i would like the write expression to write back a value two those both tags when the derived is changing.

Can i use the write expression for this of do i need to go via scripting ?

Regards

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**Author:** ![deon.korb](https://avatars.discourse-cdn.com/v4/letter/d/8c91f0/32.png) [@deon.korb](https://forum.inductiveautomation.com/u/deon.korb)\
**Post date:** [August 2, 2022, 12:47pm UTC](https://forum.inductiveautomation.com/t/use-derived-tag-write-expression-to-write-back-to-two-other-tags/62584/2 "2022-08-02T12:47:19Z")

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Maybe you can use a `runScript()` in your setter expression?

I use it quite successfully to publish to MQTT custom namespaces. Just remember to return the value from the script function, else you get a momentary null which may or may not cause you issues in the getter expression.

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<div class="post-metadata">

**Author:** ![Arnaud\_Declerck](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/arnaud_declerck/32/44872_2.png) [@Arnaud\_Declerck](https://forum.inductiveautomation.com/u/Arnaud_Declerck)\
**Post date:** [August 2, 2022, 1:46pm UTC](https://forum.inductiveautomation.com/t/use-derived-tag-write-expression-to-write-back-to-two-other-tags/62584/3 "2022-08-02T13:46:26Z")

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Hi @deon.korb,

thanks for the help the setter is an expression at my side. the getter would be a script then so i'm able to write to two tags at the same time. I forgot to mention taht i'm inside a udt.