You mention that you are contributing to your IRA "after taxes." But your IRA is a "rollover" IRA, which normally means it is a traditional IRA. This means you will deduct what you have contributed on line 32 of your taxes, reducing your tax burden. In other words, you are contributing before taxes to both your 401K and IRA. If you want to contribute after tax money (so that you don't have to pay tax when you withdraw) you should open a Roth IRA.
In terms of contributions, you can contribute to both your IRA and 401K up to the maximum amount allowable ($18,500 for the 401K and $5,500 for the IRA) assuming you have enough income to do so. If your income is sufficiently high, you may not be able to deduct the IRA contribution, though. You can look this up in the IRS table for this.
Is it a good idea to contribute to both if you aren't maxing one out? Usually one will be better than the other, but not by much. Usually employers restrict the funds you can invest in, so IRAs are often better in terms of investment options. The 401K's advantages are a little more subtle. For example, if you are sued, your 401K can't be taken away. Also your 401(k) contributions come out of the top line of your taxes, while the IRA comes out on line 32. Both types of contributions are deductible but I believe certain credits and benefits look at line 22 (total income). For most people these issues don't matter.