Replacing filename with sed in bash script

Shell Programming and Scripting
1

I need to cat two files with similar names. I am using the following script:

#!/bin/bash
if [[ -f $1 ]]
then
        file=$1
        file2="${file%R1.fastq}R2.fastq"
        echo fetching data from R2 file ...
        sleep 3
        cat $file $file2 > infile
else
        echo "Input_file passed as an argument $1 is NOT found."
        exit;
fi

However, I was expecting this to work too:

        file=$1
        file2=eval echo "$file" | sed -r 's/(.*)R1(.*)/\1R2\2/'

What am I doing wrong in my second script?
The second question would be, why I cannot use cat "${file*} to concatenate both files?
Thanks in advance for any help!

2 
file="${1%1.*}"
file2="${1/R1/R2}"
cat $file*
cat "$1" "${1/R1/R2}"
cat "$1" "$file2"
file2=$(sed 's/R1/R2/' <<<"$1")
3

You need to deploy "command substitution":

file2=$(...)

However, using eval and sed is not the smartest way to get your task done.

You cannot expect "globs" to work in a variable expansion. Use e.g.

cat ${file%R1*}*