Looking to add a second sub what do I need?

Have a type r in an mrp-m500 and want to add another with another mrp-m500 or an m1000. I have a stock 90A alternator and will up grade my battery. With m500s I'd run 8gauge and m1000 4gauge. Is there any way I can get by without doing an alternator upgrade? Maybe a cap(or 2 if 2 amps) or a second battery? Just don't want lights dimming and I'd rather not get a new alt.