Combine 2 .htaccess Files - wordpress

Question

I have 2 .htaccess files that I need to merge together. One is generated by wordpress and the other is the existing .htaccess file for the site. The 2 files are as follows:
RewriteEngine on
RewriteCond %{HTTP_HOST} !^www\.mywebsite\.com$
RewriteRule ^(.*)$ http://www.mywebsite.com/$1 [R=301]
RewriteRule ^postreview/$ /viewreview.php [NC,PT,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^.*$ ./template2.php
<IfModule mod_security.c>
# Turn off mod_security filtering.
SecFilterEngine Off
# The below probably isn't needed,
# but better safe than sorry.
SecFilterScanPOST Off
</IfModule>
php_flag session.use_trans_sid off
2nd file generated by wordpress:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /projectcars/
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /projectcars/index.php [L]
</IfModule>
I've tried several ways to combine them, but I either get a redirect error or a internal server error.

Answer 1

Here's my fix:
RewriteEngine on
RewriteCond %{HTTP_HOST} !^www.domain.com [NC]
RewriteRule (.*) http://www.domain.com/$1 [R=301,L]
RewriteBase /
RewriteRule ^postreview(\/?)$ /viewreview.php [QSA,NC,PT,L]
RewriteRule ^projectcars/index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^projectcars/(.*)$ /projectcars/index.php [L]
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule (.*) template2.php [NC,L]
<IfModule mod_security.c>
SecFilterEngine Off
SecFilterScanPOST Off
</IfModule>
php_flag session.use_trans_sid off