We are trying to concatenate two arrays, tried the concat, append to list w\\MNI

Try union({a,b,c},{1,2,3})

this returns {a,b,c,1,2,3}......thanks for the quick response though.

Oh I didn't understand correctly, you are trying to combine them. Are they of the same size?

Have a rule to append elements - the inputs will be of the type you want and single. Let this rule have the two inputs - element1 and element2 and the rule definition would be - element1&element2

Now have a second rule, where you pass your array inputs. Your second rule will look something like apply(rule!Rule1, merge(arrayInput1, arrayInput2)

For more info on the apply() and merge() functions, check out this page from the documentation:
forum.appian.com/.../Looping_Functions

I'm not following your logic....can you illustrate with a simple example like I have above? I only have two inputs and they are both arrays of the same size. Array1 is {a,b,c} and Array 2 is {1,2,3}....I'm looking for an output of {a1,b2,c3}....seems easy, but the options I've tried don't work. Thanks

Just paraphrasing what Siva has provided above.

You will need two rules. First rule's definition will be ruleInput1 & ruleInput2, where ruleInput1 and ruleInput2 are single valued inputs. Let's call this rule Rule1.

Your second rule will have the following definition - apply(rule!Rule1, merge(arrayInput1, arrayInput2). Let's call this rule Rule2

arrayInput1 and arrayInput2 are the rule inputs and can accept multiple values.

If arrayInput1 is {a,b,c} and arrayInput2 is {1, 2, 3}, calling Rule2(arraInput1, arrayInput2) should return a1,b2,c3

this works...many thanks and I hope this is useful to others.

Can you provide more details? Like how to define a rule with two parameters.

New to appian, question might be stupid

Please follow the link and create parameters as mentioned. In Appian, parameters to expression or interface rules are called rule inputs. 

https://docs.appian.com/suite/help/19.4/Expression_Rules.html#add-rule-inputs

Hope this helps.