Congratulations on your second job. Step 2 of the W-4 is for if you have multiple jobs or your Spouse Works. A few factors to take into consideration if it's as if you have 2 main jobs, or if it's 2 part-time jobs. There is a worksheet on page 3 of Form W-4 that might be beneficial in your case. https://www.irs.gov/pub/irs-pdf/fw4.pdf
Yes it is very important to check the box in Step 2 Box C. It lets them know on the payroll you have two jobs so that you will have more income to take you to a higher tax bracket. Also you could use the tax calculator called Taxcastor https://turbotax.intuit.com/tax-tools/calculators/taxcaster/ to make sure that the 2 job income keeps with what the tax calculator says. I hope this helps and great that you thought about it with 7 more months.