#JavaScript (ES6), 14 bytes

Takes input as an array of 2 strings. Inspired by Luis' answer.

JSON.stringify 

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#JavaScript (ES6),  21  20 bytes

Takes input as (a)(b).

a=>b=>[a.length,a]+b 

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Returns the length of \$a\$, followed by a comma, followed by the concatenation of \$a\$ and \$b\$.

JavaScript (ES6), 14 bytes

Takes input as an array of 2 strings. Inspired by Luis' answer.

JSON.stringify 

Try it online!

JavaScript (ES6),  21  20 bytes

Takes input as (a)(b).

a=>b=>[a.length,a]+b 

Try it online!

Returns the length of \$a\$, followed by a comma, followed by the concatenation of \$a\$ and \$b\$.

#JavaScript (ES6),  21  20 bytes

Takes input as (a)(b).

a=>b=>[a.length,a]+b 

Try it online!

Returns the length of \$a\$, followed by a comma, followed by the concatenation of \$a\$ and \$b\$.

#JavaScript (ES6), 14 bytes

Takes input as an array of 2 strings. Inspired by Luis' answer.

JSON.stringify 

Try it online!

#JavaScript (ES6),  21  20 bytes

Takes input as (a)(b).

a=>b=>[a.length,a]+b 

Try it online!

Returns the length of \$a\$, followed by a comma, followed by the concatenation of \$a\$ and \$b\$.

#JavaScript (ES6), 21 bytes

Takes input as (a)(b).

a=>b=>a.length+[,a+b] 

Try it online!

#JavaScript (ES6),  21  20 bytes

Takes input as (a)(b).

a=>b=>[a.length,a]+b 

Try it online!

Returns the length of \$a\$, followed by a comma, followed by the concatenation of \$a\$ and \$b\$.